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From my understanding, in the sodium-potassium pump we have Na+ inside the cell and K+ outside the cell, thus forming a so called "salted banana." After reading my textbook I found many statements saying that "potassium ions tend to diffuse out of the cell… sodium ions tend to flow into the cell." I initially thought that when the action potential hits Na+ leaves the cell and K+ enters the cell, which to me contradicts the previous statement.
Can someone please clarify this and differentiate between the two? I am trying to understand the action potential process as a whole.
Thank you !
Your confusion is caused by the assumption that Na+ always leaves the cell and K+ always enters. The Na+/K+ pump is there to maintain membrane potential and relative Na+ and K+ ion concentrations stable inside. When an action potential (AP) is generated, sodium channels open and sodium rushes inside to depolarize the cell( 1st phase of AP). Next, the sodium channels close and K+ begins to leave the cell (since the inside of the cell contains too many positive charges and the cell would remain depolarized if nothing was changed). Potassium leaves through leak channels. So in summary, Na+/K+ pump and the channels that are part of AP generation result in different movement of Na+ and K+ across the cell membrane.
Here is a nice review of Na+/K+ pump and action potential generation. https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/v/sodium-potassium-pump
Na + /K + -ATPase
Na⁺/K⁺-ATPase (sodium–potassium adenosine triphosphatase, also known as the Na⁺/K⁺ pump or sodium–potassium pump) is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal cells. It performs several functions in cell physiology.
The Na⁺/K⁺-ATPase enzyme is active (i.e. it uses energy from ATP). For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are imported there is hence a net export of a single positive charge per pump cycle.
The sodium–potassium pump was discovered in 1957 by the Danish scientist Jens Christian Skou, who was awarded a Nobel Prize for his work in 1997. Its discovery marked an important step forward in the understanding of how ions get into and out of cells, and it has particular significance for excitable cells such as nerve cells, which depend on this pump to respond to stimuli and transmit impulses.
All mammals have four different sodium pump sub-types, or isoforms. Each has unique properties and tissue expression patterns.  This enzyme belongs to the family of P-type ATPases.
How does the sodium-potassium pump work?
The sodium-potassium pump uses active transport to move molecules from a high concentration to a low concentration.
The sodium-potassium pump uses active transport to move molecules from a high concentration to a low concentration.
The sodium-potassium pump moves sodium ions out of and potassium ions into the cell. This pump is powered by ATP. For each ATP that is broken down, 3 sodium ions move out and 2 potassium ions move in.
In more detail:
Sodium ions bind to the pump and a phosphate group from ATP attaches to the pump, causing it to change its shape. In this new shape, the pump releases the three sodium ions and now binds two potassium ions. Once the potassium ions are bound to the pump, the phosphate group detaches. This in turn causes the pump to release the two potassium ions into the cytoplasm. The video shows this process with an animation and text.
What Is Active Transport?
Some substances can pass into or out of a cell across the plasma membrane without any energy required because they are moving from an area of higher concentration to an area of lower concentration. This type of transport is called passive transport as you learned in the last section. Other substances require energy to cross a plasma membrane often because they are moving from an area of lower concentration to an area of higher concentration. This type of transport is called active transport. The energy for active transport comes from the energy-carrying molecule called ATP (adenosine triphosphate). Active transport may also require transport proteins, such as carrier proteins, which are embedded in the plasma membrane. Two types of active transport are pump and vesicle transport.
Two pump mechanisms (primary and secondary active transports) exist for the transport of small-molecular weight material and macromolecules. The primary active transport moves ions across a membrane and creates a difference in charge across that membrane. The primary active transport system uses ATP to move a substance, such as an ion, into the cell, and often at the same time, a second substance is moved out of the cell. The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of the cell and potassium ions into the cells &mdash in all the trillions of cells in the body! Both ions are moved from areas of lower to higher concentration, so energy is needed for this "uphill" process. The energy is provided by ATP. The sodium-potassium pump also requires carrier proteins. Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Figure (PageIndex<2>) shows in greater detail how the sodium-potassium pump works and the specific roles played by carrier proteins in this process.
Figure (PageIndex<2>): The sodium-potassium pump. The sodium-potassium pump moves sodium ions (Na(^<+>)) out of the cell and potassium ions (K(^<+>)) into the cell. First, three sodium ions bind with a carrier protein in the cell membrane. Then, the carrier protein receives a phosphate group from ATP. When ATP loses a phosphate group, energy is released. The carrier protein changes shape, and as it does, it pumps the three sodium ions out of the cell. At that point, two potassium ions bind to the carrier protein. The process is reversed, and the potassium ions are pumped into the cell.
To appreciate the importance of the sodium-potassium pump, you need to know more about the roles of sodium and potassium in the body. Both are essential dietary minerals, meaning you have to obtain them in the foods you eat. Both sodium and potassium are also electrolytes, meaning that they dissociate into ions (charged particles) in solution, which allows them to conduct electricity. Normal body functions require a very narrow range of concentrations of sodium and potassium ions in body fluids, both inside and outside of cells.
- Sodium is the principal ion in the fluid outside of cells. Normal sodium concentrations are about 10 times higher outside than inside of cells.
- Potassium is the principal ion in the fluid inside of cells. Normal potassium concentrations are about 30 times higher inside than outside of cells.
These differences in concentration create an electrical gradient across the cell membrane, called the membrane potential. the secondary active transport describes the movement of material using the energy of the electrochemical gradient established by the primary active transport. Using the energy of the electrochemical gradient created by the primary active transport system, other substances such as amino acids and glucose can be brought into the cell through membrane channels. ATP itself is formed through secondary active transport using a hydrogen ion gradient in the mitochondrion. Tightly controlling the membrane potential is critical for vital body functions, including the transmission of nerve impulses and the contraction of muscles. A large percentage of the body's energy goes to maintaining this potential across the membranes of its trillions of cells with the sodium-potassium pump.
Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport. Vesicle transport requires energy, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis.
Endocytosis is a type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. It is used by all cells of the body because most substances important to them are polar and consist of big molecules, and thus cannot pass through the hydrophobic plasma membrane. When an entire cell or other solid particle is engulfed, the process is called phagocytosis. When fluid is engulfed, the process is called pinocytosis. When the content is taken in specifically with the help of receptors on the plasma membrane, it is called receptor-mediated endocytosis.
Figure (PageIndex<3>): (On the right) Phagocytosis is when the plasma membrane wraps around a solid particle outside the cell using projections called pseudopodia. The membrane then pinches off to form a phagosome (food vacuole). (Middle)Pinocytosis is when the membrane folds to form a vesicle that carries substances dissolved in the extracellular fluid. (On the left) Receptor-mediated endocytosis occurs when the receptors on the plasma membrane bind to specific particles. The coated pit region of the membrane forms a coated vesicle containing the receptors with their bound particles.
A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specific for certain substances. The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell. If passage across the membrane of the target of receptor-mediated endocytosis is ineffective, it will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by a failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as &ldquobad&rdquo cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood because their cells cannot clear the chemical from their blood.
Exocytosis is a type of vesicle transport that moves a substance out of the cell. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell.
Figure (PageIndex<4>): Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.
Modulation of the Na,K-ATPase by Magnesium Ions
Since the beginning of investigations of the Na,K-ATPase, it has been well-known that Mg 2+ is an essential cofactor for activation of enzymatic ATP hydrolysis without being transported through the cell membrane. Moreover, experimental evidence has been collected through the years that shows that Mg 2+ ions have a regulatory effect on ion transport by interacting with the cytoplasmic side of the ion pump. Our experiments allowed us to reveal the underlying mechanism. Mg 2+ is able to bind to a site outside the membrane domain of the protein's α subunit, close to the entrance of the access channel to the ion-binding sites, thus modifying the local concentration of the ions in the electrolyte, of which Na + , K + , and H + are of physiological interest. The decrease in the concentration of these cations can be explained by electrostatic interaction and estimated by the Debye-Hückel theory. This effect provokes the observed apparent reduction of the binding affinity of the binding sites of the Na,K-ATPase in the presence of various Mg 2+ concentrations. The presence of the bound Mg 2+ , however, does not affect the reaction kinetics of the transport function of the ion pump. Therefore, stopped-flow experiments could be performed to gain the first insight into the Na + binding kinetics on the cytoplasmic side by Mg 2+ concentration jump experiments.
Functions of the sodium-potassium pump:
The sodium-potassium pump is an essential cellular membrane protein that functions by pumping out three sodium ions and taking. In two potassium ions. This mechanism preserves the electrochemical gradient formed from the varying concentrations of sodium and potassium ions within the cell and its exterior.
The sodium-potassium pump, also called Na, K-ATPase, is responsible for active transportation. This procedure demands energy to transfer the sodium and also potassium ions into and away from the cellular materials. Adenosine triphosphate, or ATP, is the high-energy transporting molecule which can be the chief approach of obtaining this required energy. Whenever ATP endures hydrolysis, the energy distributed from its bonds changes the shape and also the composition of the sodium-potassium pump. Allowing the pump to proceed with the sodium and also potassium ions across cell membranes.
Learn All Points About Sodium Potassium Pump (Na+K+ATPase) and Concept About Resting Membrane Potential With Details
Three sodium particles from inside the cell first tie to the vehicle protein. At that point a phosphate bunch is moved from ATP to the vehicle protein making it change shape and delivery the sodium particles outside the cell. Two potassium particles from outside the cell at that point tie to the vehicle protein and as the phosphate is taken out, the protein expects its unique shape and deliveries the potassium particles inside the cell.
On the off chance that the siphon was to proceed with unchecked there would be no sodium or potassium particles left to siphon, however there are likewise sodium and potassium particle diverts in the layer. These channels are regularly shut, yet in any event, when shut, they “spill”, permitting sodium particles to spill in and potassium particles to spill out, down their individual fixation inclinations.
Centralization of Particles Inside and Outside the Neuron Very Still:
Ion Concentration inside cell/mmol dm-3 Concentration outside cell/mmol dm-3 Why don’t the particles descend their fixation slope?
K+ 150.0 2.5 K+ particles don’t move out of the neuron down their focus angle because of a development of positive charges outside the layer. This repulses the development of any more K+ particles out of the cell.
The chloride particles don’t move into the cytoplasm as the adversely charged protein atoms that can’t cross the surface layer repulse them.
The blend of the Na+K+ATPase siphon and the hole channels cause a steady awkwardness of Na+ and K+ particles over the film. This awkwardness of particles causes an expected distinction (or voltage) between within the neuroma and its environmental factors, called the resting layer potential. The film potential is consistently negative inside the cell, and shifts in size from – 20 to – 200 mV (milivolt) in various cells and species (in people it is – 70mV).
The Na+K+ATPase is thought to have developed as an osmoregulatory to keep the inside water possible high thus stop water entering creature cells and blasting them. Plant cells needn’t bother with this as they have solid cells dividers to forestall blasting.
Sodium-Potassium Pump Explained
At this very moment, there is a diversified network of nerve impulses running throughout the human anatomy. However, none of these complex movements are possible without the help of the sodium-potassium pump because it is specifically designed to transport proteins that are found within the cell membranes. Cell membranes are semi-permeable external barriers of majority of cells inside the body.
The primary function of the sodium-potassium pump is to propel potassium ions inside the cell, and at the same time, extracting sodium ions from the cell. Due to this intricacy, the sodium-potassium pump is hailed as one of the most critical processes inside the body for without it, electrical signals will not be possible, and the cells will eventually deteriorate.
The sodium-potassium pump is notable in nerve cells, in the kidneys, and also plays an important role in heart contractions and blood pressure. One must thank the sodium-potassium pump for a steady heartbeat.
Depolarization in Cardiac Muscles
Like skeletal and smooth muscles, the depolarization in cardiac muscles is also coupled with their contraction. These muscles are excited by the action potential traveling in the conductive system of the heart.
The cardiac muscles are connected to each other as well as the cells of the heart’s conductive system via gap junctions. These gap junctions serve as electrical synapses allowing the ions to easily flow between the cells.
When these muscles are excited, sodium channels are opened. The influx of sodium ions into the cells causes depolarization spike in these muscles. Once the spike is over, the potassium channels are opened. In the meantime, the calcium channels also open, resulting in the plateau phase of depolarization. This phase is over when the calcium channels close.
Thus, in the case of cardiac muscle, depolarization is caused by the influx of sodium as well as calcium ions. The calcium ions flowing into the cell during the plateau stage are required for the contraction process. In addition, depolarization of the cell also releases calcium ions from the sarcoplasmic reticulum. These calcium ions are also needed for the contraction of cardiac muscles.
The combined gradient that affects an ion includes its concentration gradient and its electrical gradient. A positive ion, for example, might tend to diffuse into a new area, down its concentration gradient, but if it is diffusing into an area of net positive charge, its diffusion will be hampered by its electrical gradient. When dealing with ions in aqueous solutions, a combination of the electrochemical and concentration gradients, rather than just the concentration gradient alone, must be considered. Living cells need certain substances that exist inside the cell in concentrations greater than they exist in the extracellular space. Moving substances up their electrochemical gradients requires energy from the cell. Active transport uses energy stored in ATP to fuel this transport. Active transport of small molecular-sized materials uses integral proteins in the cell membrane to move the materials: These proteins are analogous to pumps. Some pumps, which carry out primary active transport, couple directly with ATP to drive their action. In co-transport (or secondary active transport), energy from primary transport can be used to move another substance into the cell and up its concentration gradient.